About how much of the human body is water?

The human body has a variable water content that changes daily and with age and gender and body type.

On average, adult humans have 57 to 63 % water by mass.

At birth, a baby can have up to 75% water weight and this decreases with age.
Obese individuals can have as little as 45% water weight.
Women tend to have a few percent less water weight than men, but this is largely a difference in muscle and body fat between individuals.